The majority of oil and gas reserves are located thousands of feet beneath the surface of the earth in a variety of subterranean formations. The primary goal of the oil and gas industry is to locate, access, and produce these reserves in an economic fashion. In order to access and economically produce these reserves the oil and gas industry relies upon technologies that can perform various tasks in the remote and hostile environment characteristic of subterranean formations. Examples of such tasks are, drilling, perforating, stimulating, logging, coring, fluid sampling, etc. Most remote tasks or processes are expensive, require numerous operations, rely upon skilled operators, and require an appreciable quantity of specialized equipment to achieve the desired goal. Typically, most of the expense associated with remote access is related to the amount of time that specialized equipment and trained personnel must be utilized to perform the required tasks. As a result, technologies that enable rapid, effective, and reliable remote operations increase the economic gains attainable from a given reserve by reducing the time required for remote access. The process of reservoir stimulation will be expounded upon in the forthcoming discussion to illustrate the complexities associated with remote access, and to introduce the gains attainable by applying the proposed invention to the remote access task of stimulation.
When a hydrocarbon-bearing, subterranean reservoir formation does not have enough permeability or flow capacity for the hydrocarbons to flow to the surface in economic quantities or at optimum rates, hydraulic fracturing or chemical (usually acid) stimulation is often used to increase the flow capacity. A wellbore penetrating a subterranean formation typically consists of a metal pipe (casing) cemented into the original drill hole. Holes (perforations) are placed to penetrate through the casing and the cement sheath surrounding the casing to allow hydrocarbon flow into the wellbore and, if necessary, to allow treatment fluids to flow from the wellbore into the formation.
Hydraulic fracturing consists of injecting fluids (usually viscous shear thinning, non-Newtonian gels or emulsions) into a formation at such high pressures and rates that the reservoir rock fails and forms a plane, typically vertical, fracture (or fracture network) much like the fracture that extends through a wooden log as a wedge is driven into it. Granular proppant material, such as sand, ceramic beads, or other materials, is generally injected with the later portion of the fracturing fluid to hold the fracture(s) open after the pressure is released. Increased flow capacity from the reservoir results from the flow path left between grains of the proppant material within the fracture(s). In chemical stimulation treatments, flow capacity is improved by dissolving materials in the formation or otherwise changing formation properties.
Application of hydraulic fracturing as described above is a routine part of petroleum industry operations as applied to individual target zones of up to about 60 meters (200 feet) of gross, vertical thickness of subterranean formation. When there are multiple or layered reservoirs to be hydraulically fractured, or a very thick hydrocarbon-bearing formation (over about 60 meters), then alternate treatment techniques are required to obtain treatment of the entire target zone.
When multiple hydrocarbon-bearing zones are stimulated by hydraulic fracturing or chemical stimulation treatments, economic and technical gains are realized by injecting multiple treatment stages that can be diverted (or separated) by various means, including mechanical devices such as bridge plugs, packers, downhole valves, sliding sleeves, and baffle/plug combinations; ball sealers; particulates such as sand, ceramic material, proppant, salt, waxes, resins, or other compounds; or by alternative fluid systems such as viscosified fluids, gelled fluids, foams, or other chemically formulated fluids; or using limited entry methods.
In mechanical bridge plug diversion, for example, the deepest interval is first perforated and fracture stimulated, then the interval is typically isolated by a wireline-set bridge plug, and the process is repeated in the next interval up. Assuming ten target perforation intervals, treating 300 meters (1,000 feet) of formation in this manner would typically require ten jobs over a time interval of ten days to two weeks with not only multiple fracture treatments, but also multiple perforating and bridge plug running operations. At the end of the treatment process, a wellbore clean-out operation would be required to remove the bridge plugs and put the well on production. The major advantage of using bridge plugs or other mechanical diversion agents is high confidence that the entire target zone is treated. The major disadvantages are the high cost of treatment resulting from multiple trips into and out of the wellbore and the risk of complications resulting from so many operations in the well. For example, a bridge plug can become stuck in the casing and need to be drilled out at great expense. A further disadvantage is that the required wellbore clean-out operation may damage some of the successfully fractured intervals.
To overcome some of the limitations associated with completion operations that require multiple trips of hardware into and out of the wellbore to perforate and stimulate subterranean formations, methods and apparatus have been proposed for “single-trip” deployment of a downhole tool assembly to allow for fracture stimulation of zones in conjunction with perforating. Specifically, these methods and apparatus allow operations that minimize the number of required wellbore operations and time required to complete these operations, thereby reducing the stimulation treatment cost. The tool strings used for these types of applications can be very long and the tool must complete a large number of tasks in a remote downhole environment. The tool string hardware that is assembled to complete these downhole tasks is generally referred to as a bottom hole assembly or “BHA.”
An apparatus and method is needed that: 1) independently performs numerous operations downhole; 2) independently performs the operations in a preprogrammed logical sequence; 3) independently performs the operations at the proper time; 4) uses pressure as the primary basis for control and actuation; 5) is capable of numerous independent cycles in a single trip; 6) eliminates the need for operator interaction; and 7) provides the flexibility to incorporate the most reliable and proven hardware designs (annular or non-annular based designs). The result would be a highly reliable intelligent BHA capable of single trip multi-use remote access with little or no surface interaction, essentially a pressure driven downhole computer or downhole brain.